Fundamentals of Soil Science Soil Organic Matter

Lecture 6 SOM’s Influence on Soil Properties and Plants

Learning Objectives Lecture 6 – Identify factors that lead to a loss or gain of organic matter in soils Explain the conundrum of soil organic matter management List five guidelines for managing soil organic matter Discuss changes in active and passive pools of organic matter as a result of management Name the greenhouse gases of importance to soil processes and the relative warming potential of each

Lecture 6 - Topics Factors controlling the level of soil organic matter Major soil C pools Maintenance of soil organic matter Summary and review

Carbon Inputs – Outputs = Storage Plants Litter Soil organic matter Gains in carbon come from plant residues and applied organic materials Losses in carbon are due to respiration (CO2 losses), plant removals, and erosion.

Factors promoting gains Factors promoting losses Balance of Carbon Factors Affecting the Balance Between Gains and Losses or Organic Matter in Soils Factors promoting gains Factors promoting losses Green manures or cover crops Conservation tillage Return of plant residues Low temperatures and shading Controlled grazing High soil moisture Surface mulches Application of compost and manures Appropriate nitrogen levels High plant productivity High plant root:shoot ratio Erosion Intensive tillage Whole plant removal High temperatures and exposure to sun Overgrazing Low soil moisture Fire Application of only inorganic materials Excessive mineral nitrogen Low plant productivity Low plant root:shoot ratio

Managing SOM Management of soil organic matter leads to reduction in greenhouse gas emission or to enhanced soil quality and plant production

Conundrum – SOM must simultaneously decompose and accumulate. SOM must decompose to become a source of nutrients for plants and organic compounds that promote biological diversity, disease suppression, aggregate stability and metal chelation. SOM must accumulate for these same functions as well as for sequestering of C, enhancement of soil water-holding, adsorption of exchangeable cations, immobilization of pesticides and detoxification of metals.

General Guidelines for Managing SOM Continuous supply of plant residues

General Guidelines for Managing SOM Continuous supply of plant residues Each system has its own “ideal” level of SOM

General Guidelines for Managing SOM Microbial activity, CO2 evolved Nitrate depression period Soluble N level in soil C/N ratio of residues Residues added Time C/N ratio 60 40 20 80 (a) (b) Continuous supply of plant residues Each system has its own “ideal” level of SOM Adequate N is requisite

General Guidelines for Managing SOM Continuous supply of plant residues Each system has its own “ideal” level of SOM Adequate N is requisite Tillage should be reduced or eliminated

General Guidelines for Managing SOM Continuous supply of plant residues Each system has its own “ideal” level of SOM Adequate N is requisite Tillage should be reduced or eliminated Encourage perennial vegetation and natural ecosystems

Pools of SOM Small % of residue is retained Plant residues Structural C high lignin, low N 2-4 years C/N=100-200 Metabolic C low lignin, high N 0.1-0.5 year C/N=10-25 Small % of residue is retained Offset by slow decomposition Often in equilibrium in mature ecosystems Disturbance can cause drastic change CO2 CO2 Active SOM 1-2 years C/N = 15-30 CO2 Slow SOM 15-100 years C/N = 10-25 CO2 Passive SOM 500-5000 years C/N = 7-10 CO2

SOM Active Pool Active Pool - 10-20% of SOM – labile materials with half-lives of only a few days to a few years. Provides most of the accessible food for soil organisms and most of the readily mineralizable nitrogen. Beneficial effects on structural stability that lead to enhanced infiltration of water, erosion resistance, ease of tillage.

SOM Slow Pool Slow Pool – Between Active and Passive pools Particulate matter high in lignin and other slowly decomposable and chemically resistant components. (Half-lives in decades) Source of mineralizable N, P, and S Important source of mineralized nitrogen and provides food source for k-strategist microbes.

SOM Passive Pool Passive Pool – 60-90 % of SOM – materials remaining in soil for hundreds or thousands of years. Material physically protected in clay-humus complexes Responsible for cation exchange and water-holding capacities contributed to soil by organic matter Composed of humic substances

Pools of SOM (cont.)

Changes in Active and Passive Pools with Soil Management Monitoring the Active C Pool can serve as an early warning of soil quality changes The Active Pool reflects the greatest change in organic matter, either loss through cultivation or gain through addition of organic material.

Global Climate Change Levels of certain gases in Earth’s atmosphere cause concern Carbon dioxide, methane, nitrous oxide, ozone, chlorofluorocarbons (CFCs) Greenhouse gases (GHG) trap much of the outgoing long-wavelength radiation GHG produced by biological processes, such as those occurring in soil, account for ½ of the rising greenhouse effect. Root respiration, decomposition of exudates and SOM produce CO2 Methanogenesis produces CH4 Nitrification and denitrification produce N2O

Global Warming Potential N2O and CH4 are present in lower concentrations than CO2 Their potential to trap infrared radiation is greater GWP of N2O is 298 x and CH4 is 25 x CO2 over 100 years Small increases in the production of these trace gases impacts net emissions of an ecosystem or production system

GHG Emission from Soil

Trace Gas Emission in CO2 Equivalents Sugar cane Napier grass

Renewable Energy: Biofuels

Summary SOM is beneficial to soil biological, physical, and chemical properties To realize this potential you must build SOM up, but also have mineralization, in balance Management can have enormous impact particularly on the active soil C pools Trace GHG that originate from soils, such as CH4 and N2O have disproportionate effects on climate change compared to CO2